JACKS

Jack is a useful tools which provide to lift heavy burden up or change position of motionless substances by appling pressure. Jacks which have little capacity are used in machine tools and cars, jacks which hane big capacity are used in heavy machines and lifting the other burdens.

Jacks can be classified in two parts as mechanical jacks and hydraulic jacks. Moreover mechanical jacks have some various sub-groups such as screw jack, toothed rock jack etc. Hydraulic jacks have some various as mechanical jacks. For example bottle type hydraulic jacks, horizontal cars hydraulic jacks etc.

Screw jacks are formed by big and sequare or crooked chamfered steel screw. The screw usually screw into the nut that can turn freely at tip of the pipe. Ifthe nut is turned by simple or nailed handle the screw will go out from inside of thepipe. As a result of this, screw and the tip of the pipe will become distant from each other. The tips usually holds with lining onto the surfaces. So the apperture which is between the surfaces will increase. There is no need another system to stabilized irreversible system, because the step of screw is too small. The nut is constant in other screw jack types and so the screw is turned. Some of these are telescopic becausethe screws can go into each others. As a result higher lifting height can be provided based on tools own height. Screw jacks are used especially to lift burden up support the walls temporarly.

Toothed rock jacks are formed by toothed rock that are activated by the gearing. The fuselage of jack can be wooden or metal. On the top part of the toothed rocks, there is a couple of hook, and on the under partof the toothed rock founded a foot that can be put under the burden that is hanging on a few meters from the ground. There are two types of tuuthed rock jacks. One of is that fuselage of jack is constant and it’s toothed rocks are rising, the other type is that the fuselage of jacks are rising. Toothed rock is on the ground at the jacks that their fuselage are rising and the fuselage carries the burden.

Hydraulic jacks are made by a little diameter cylinder and a bid diameter cylinder. The burden on the big diameter cylinder can be lifted by appling afew pressure on the the little diameter cylinder. If the burden that is lifed up, wants to get down, liquid in the big diameter cylinder sends into water tank that is connected the big diameter cylinder by opening a valve. Hydraulic jacks can have huge capacity but increasing the capacity of jacks make the transportation harder.

Robots and Artificial Intelligence

Artificial intelligence (AI) is arguably the most exciting field in robotics. It's certainly the most controversial: Everybody agrees that a robot can work in an assembly line, but there's no consensus on whether a robot can ever be intelligent.

Like the term "robot" itself, artificial intelligence is hard to define. Ultimate AI would be a recreation of the human thought process -- a man-made machine with our intellectual abilities. This would include the ability to learn just about anything, the ability to reason, the ability to use language and the ability to formulate original ideas. Roboticists are nowhere near achieving this level of artificial intelligence, but they have had made a lot of progress with more limited AI. Today's AI machines can replicate some specific elements of intellectual ability.

Computers can already solve problems in limited realms. The basic idea of AI problem-solving is very simple, though its execution is complicated. First, the AI robot or computer gathers facts about a situation through sensors or human input. The computer compares this information to stored data and decides what the information signifies. The computer runs through various possible actions and predicts which action will be most successful based on the collected information. Of course, the computer can only solve problems it's programmed to solve -- it doesn't have any generalized analytical ability. Chess computers are one example of this sort of machine.

Some modern robots also have the ability to learn in a limited capacity. Learning robots recognize if a certain action (moving its legs in a certain way, for instance) achieved a desired result (navigating an obstacle). The robot stores this information and attempts the successful action the next time it encounters the same situation. Again, modern computers can only do this in very limited situations. They can't absorb any sort of information like a human can. Some robots can learn by mimicking human actions. In Japan, roboticists have taught a robot to dance by demonstrating the moves themselves.

Some robots can interact socially. Kismet, a robot at M.I.T's Artificial Intelligence Lab, recognizes human body language and voice inflection and responds appropriately. Kismet's creators are interested in how humans and babies interact, based only on tone of speech and visual cue. This low-level interaction could be the foundation of a human-like learning system.

Kismet and other humanoid robots at the M.I.T. AI Lab operate using an unconventional control structure. Instead of directing every action using a central computer, the robots control lower-level actions with lower-level computers. The program's director, Rodney Brooks, believes this is a more accurate model of human intelligence. We do most things automatically; we don't decide to do them at the highest level of consciousness.

The real challenge of AI is to understand how natural intelligence works. Developing AI isn't like building an artificial heart -- scientists don't have a simple, concrete model to work from. We do know that the brain contains billions and billions of neurons, and that we think and learn by establishing electrical connections between different neurons. But we don't know exactly how all of these connections add up to higher reasoning, or even low-level operations. The complex circuitry seems incomprehensible.

Because of this, AI research is largely theoretical. Scientists hypothesize on how and why we learn and think, and they experiment with their ideas using robots. Brooks and his team focus on humanoid robots because they feel that being able to experience the world like a human is essential to developing human-like intelligence. It also makes it easier for people to interact with the robots, which potentially makes it easier for the robot to learn.

Just as physical robotic design is a handy tool for understanding animal and human anatomy, AI research is useful for understanding how natural intelligence works. For some roboticists, this insight is the ultimate goal of designing robots. Others envision a world where we live side by side with intelligent machines and use a variety of lesser robots for manual labor, health care and communication. A number of robotics experts predict that robotic evolution will ultimately turn us into cyborgs -- humans integrated with machines. Conceivably, people in the future could load their minds into a sturdy robot and live for thousands of years!

In any case, robots will certainly play a larger role in our daily lives in the future. In the coming decades, robots will gradually move out of the industrial and scientific worlds and into daily life, in the same way that computers spread to the home in the 1980s.

Homebrew Robots

In the last couple of sections, we looked at the most prominent fields in the world of robots -- industry robotics and research robotics. Professionals in these fields have made most of the major advancements in robotics over the years, but they aren't the only ones making robots. For decades, a small but passionate band of hobbyists has been creating robots in garages and basements all over the world.

Homebrew robotics is a rapidly expanding subculture with a sizable Web presence. Amateur roboticists cobble together their creations using commercial robot kits, mail order components, toys and even old VCRs.

Homebrew robots are as varied as professional robots. Some weekend roboticists tinker with elaborate walking machines, some design their own service bots and others create competitive robots. The most familiar competitive robots are remote control fighters like you might see on "BattleBots." These machines aren't considered "true robots" because they don't have reprogrammable computer brains. They're basically souped-up remote control cars.

More advanced competitive robots are controlled by computer. Soccer robots, for example, play miniaturized soccer with no human input at all. A standard soccer bot team includes several individual robots that communicate with a central computer. The computer "sees" the entire soccer field with a video camera and picks out its own team members, the opponent's members, the ball and the goal based on their color. The computer processes this information at every second and decides how to direct its own team.

Check out the official RoboCup Web site for more information on Soccer robots, and Google > Computers > Robotics > Competitions for information on other robot competitions. Google > Computers > Robotics > Building will give you more information on building your own robots.

Adaptable and Universal
The personal computer revolution has been marked by extraordinary adaptability. Standardized hardware and programming languages let computer engineers and amateur programmers mold computers to their own particular purposes. Computer components are sort of like art supplies -- they have an infinite number of uses.

Most robots to date have been more like kitchen appliances. Roboticists build them from the ground up for a fairly specific purpose. They don't adapt well to radically new applications.

This situation may be changing. A company called Evolution Robotics is pioneering the world of adaptable robotics hardware and software. The company hopes to carve out a niche for itself with easy-to-use "robot developer kits."

The kits come with an open software platform tailored to a range of common robotic functions. For example, roboticists can easily give their creations the ability to follow a target, listen to voice commands and maneuver around obstacles. None of these capabilities are revolutionary from a technology standpoint, but it's unusual that you would find them in one simple package.

The kits also come with common robotics hardware that connects easily with the software. The standard kit comes with infrared sensors, motors, a microphone and a video camera. Roboticists put all these pieces together with a souped-up erector set -- a collection of aluminum body pieces and sturdy wheels.

These kits aren't your run-of-the-mill construction sets, of course. At upwards of $700, they're not cheap toys. But they are a big step toward a new sort of robotics. In the near future, creating a new robot to clean your house or take care of your pets while you're away might be as simple as writing a BASIC program to balance your checkbook.

Autonomous Robots

Autonomous robots can act on their own, independent of any controller. The basic idea is to program the robot to respond a certain way to outside stimuli. The very simple bump-and-go robot is a good illustration of how this works.

This sort of robot has a bumper sensor to detect obstacles. When you turn the robot on, it zips along in a straight line. When it finally hits an obstacle, the impact pushes in its bumper sensor. The robot's programming tells it to back up, turn to the right and move forward again, in response to every bump. In this way, the robot changes direction any time it encounters an obstacle.

Advanced robots use more elaborate versions of this same idea. Roboticists create new programs and sensor systems to make robots smarter and more perceptive. Today, robots can effectively navigate a variety of environments.

Simpler mobile robots use infrared or ultrasound sensors to see obstacles. These sensors work the same way as animal echolocation: The robot sends out a sound signal or a beam of infrared light and detects the signal's reflection. The robot locates the distance to obstacles based on how long it takes the signal to bounce back.

More advanced robots use stereo vision to see the world around them. Two cameras give these robots depth perception, and image-recognition software gives them the ability to locate and classify various objects. Robots might also use microphones and smell sensors to analyze the world around them.

Some autonomous robots can only work in a familiar, constrained environment. Lawn-mowing robots, for example, depend on buried border markers to define the limits of their yard. An office-cleaning robot might need a map of the building in order to maneuver from point to point.

More advanced robots can analyze and adapt to unfamiliar environments, even to areas with rough terrain. These robots may associate certain terrain patterns with certain actions. A rover robot, for example, might construct a map of the land in front of it based on its visual sensors. If the map shows a very bumpy terrain pattern, the robot knows to travel another way. This sort of system is very useful for exploratory robots that operate on other planets (check out JPL Robotics to learn more).

An alternative robot design takes a less structured approach -- randomness. When this type of robot gets stuck, it moves its appendages every which way until something works. Force sensors work very closely with the actuators, instead of the computer directing everything based on a program. This is something like an ant trying to get over an obstacle -- it doesn't seem to make a decision when it needs to get over an obstacle, it just keeps trying things until it gets over it.

Mobile Robots

Robotic arms are relatively easy to build and program because they only operate within a confined area. Things get a bit trickier when you send a robot out into the world.

The first obstacle is to give the robot a working locomotion system. If the robot will only need to move over smooth ground, wheels or tracks are the best option. Wheels and tracks can also work on rougher terrain if they are big enough. But robot designers often look to legs instead, because they are more adaptable. Building legged robots also helps researchers understand natural locomotion -- it's a useful exercise in biological research.
Typically, hydraulic or pneumatic pistons move robot legs back and forth. The pistons attach to different leg segments just like muscles attach to different bones. It's a real trick getting all these pistons to work together properly. As a baby, your brain had to figure out exactly the right combination of muscle contractions to walk upright without falling over. Similarly, a robot designer has to figure out the right combination of piston movements involved in walking and program this information into the robot's computer. Many mobile robots have a built-in balance system (a collection of gyroscopes, for example) that tells the computer when it needs to correct its movements

Bipedal locomotion (walking on two legs) is inherently unstable, which makes it very difficult to implement in robots. To create more stable robot walkers, designers commonly look to the animal world, specifically insects. Six-legged insects have exceptionally good balance, and they adapt well to a wide variety of terrain.

Some mobile robots are controlled by remote -- a human tells them what to do and when to do it. The remote control might communicate with the robot through an attached wire, or using radio or infrared signals. Remote robots, often called puppet robots, are useful for exploring dangerous or inaccessible environments, such as the deep sea or inside a volcano. Some robots are only partially controlled by remote. For example, the operator might direct the robot to go to a certain spot, but not steer it there -- the robot would find its own way.

The Robotic Arm

The term robot comes from the Czech word robota, generally translated as "forced labor." This describes the majority of robots fairly well. Most robots in the world are designed for heavy, repetitive manufacturing work. They handle tasks that are difficult, dangerous or boring to human beings.

The most common manufacturing robot is the robotic arm. A typical robotic arm is made up of seven metal segments, joined by six joints. The computer controls the robot by rotating individual step motors connected to each joint (some larger arms use hydraulics or pneumatics). Unlike ordinary motors, step motors move in exact increments (check out Anaheim Automation to find out how). This allows the computer to move the arm very precisely, repeating exactly the same movement over and over again. The robot uses motion sensors to make sure it moves just the right amount.

An industrial robot with six joints closely resembles a human arm -- it has the equivalent of a shoulder, an elbow and a wrist. Typically, the shoulder is mounted to a stationary base structure rather than to a movable body. This type of robot has six degrees of freedom, meaning it can pivot in six different ways. A human arm, by comparison, has seven degrees of freedom.

Your arm's job is to move your hand from place to place. Similarly, the robotic arm's job is to move an end effector from place to place. You can outfit robotic arms with all sorts of end effectors, which are suited to a particular application. One common end effector is a simplified version of the hand, which can grasp and carry different objects. Robotic hands often have built-in pressure sensors that tell the computer how hard the robot is gripping a particular object. This keeps the robot from dropping or breaking whatever it's carrying. Other end effectors include blowtorches, drills and spray painters.

Industrial robots are designed to do exactly the same thing, in a controlled environment, over and over again. For example, a robot might twist the caps onto peanut butter jars coming down an assembly line. To teach a robot how to do its job, the programmer guides the arm through the motions using a handheld controller. The robot stores the exact sequence of movements in its memory, and does it again and again every time a new unit comes down the assembly line.

Most industrial robots work in auto assembly lines, putting cars together. Robots can do a lot of this work more efficiently than human beings because they are so precise. They always drill in the exactly the same place, and they always tighten bolts with the same amount of force, no matter how many hours they've been working. Manufacturing robots are also very important in the computer industry. It takes an incredibly precise hand to put together a tiny microchip.

Robot Basics

The vast majority of robots do have several qualities in common. First of all, almost all robots have a movable body. Some only have motorized wheels, and others have dozens of movable segments, typically made of metal or plastic. Like the bones in your body, the individual segments are connected together with joints.

Robots spin wheels and pivot jointed segments with some sort of actuator. Some robots use electric motors and solenoids as actuators; some use a hydraulic system; and some use a pneumatic system (a system driven by compressed gases). Robots may use all these actuator types.

A robot needs a power source to drive these actuators. Most robots either have a battery or they plug into the wall. Hydraulic robots also need a pump to pressurize the hydraulic fluid, and pneumatic robots need an air compressor or compressed air tanks.

The actuators are all wired to an electrical circuit. The circuit powers electrical motors and solenoids directly, and it activates the hydraulic system by manipulating electrical valves. The valves determine the pressurized fluid's path through the machine. To move a hydraulic leg, for example, the robot's controller would open the valve leading from the fluid pump to a piston cylinder attached to that leg. The pressurized fluid would extend the piston, swiveling the leg forward. Typically, in order to move their segments in two directions, robots use pistons that can push both ways.

The robot's computer controls everything attached to the circuit. To move the robot, the computer switches on all the necessary motors and valves. Most robots are reprogrammable -- to change the robot's behavior, you simply write a new program to its computer.

Not all robots have sensory systems, and few have the ability to see, hear, smell or taste. The most common robotic sense is the sense of movement -- the robot's ability to monitor its own motion. A standard design uses slotted wheels attached to the robot's joints. An LED on one side of the wheel shines a beam of light through the slots to a light sensor on the other side of the wheel. When the robot moves a particular joint, the slotted wheel turns. The slots break the light beam as the wheel spins. The light sensor reads the pattern of the flashing light and transmits the data to the computer. The computer can tell exactly how far the joint has swiveled based on this pattern. This is the same basic system used in computer mice.

These are the basic nuts and bolts of robotics. Roboticists can combine these elements in an infinite number of ways to create robots of unlimited complexity. In the next section, we'll look at one of the most popular designs, the robotic arm.

Robots

On the most basic level, human beings are made up of five major components:

• A body structure
• A muscle system to move the body structure
• A sensory system that receives information about the body and the surrounding environment
• A power source to activate the muscles and sensors
• A brain system that processes sensory information and tells the muscles what to do

Of course, we also have some intangible attributes, such as intelligence and morality, but on the sheer physical level, the list above about covers it.

A robot is made up of the very same components. A typical robot has a movable physical structure, a motor of some sort, a sensor system, a power supply and a computer "brain" that controls all of these elements. Essentially, robots are man-made versions of animal life -- they are machines that replicate human and animal behavior.

In this article, we'll explore the basic concept of robotics and find out how robots do what they do.

Joseph Engelberger, a pioneer in industrial robotics, once remarked "I can't define a robot, but I know one when I see one." If you consider all the different machines people call robots, you can see that it's nearly impossible to come up with a comprehensive definition. Everybody has a different idea of what constitutes a robot.

You've probably heard of several of these famous robots:

• R2D2 and C-3PO: The intelligent, speaking robots with loads of personality in the "Star Wars" movies
• Sony's AIBO: A robotic dog that learns through human interaction
• Honda's ASIMO: A robot that can walk on two legs like a person
• Industrial robots: Automated machines that work on assembly lines
• Data: The almost human android from "Star Trek"
• BattleBots: The remote control fighters on Comedy Central
• Bomb-defusing robots
• NASA's Mars rovers
• HAL: The ship's computer in Stanley Kubrick's "2001: A Space Odyssey"
• Robomower: The lawn-mowing robot from Friendly Robotics
• The Robot in the television series "Lost in Space"
• MindStorms: LEGO's popular robotics kit
  

All of these things are considered robots, at least by some people. The broadest definition around defines a robot as anything that a lot of people recognize as a robot. Most roboticists (people who build robots) use a more precise definition. They specify that robots have a reprogrammable brain (a computer) that moves a body.

By this definition, robots are distinct from other movable machines, such as cars, because of their computer element. Many new cars do have an onboard computer, but it's only there to make small adjustments. You control most elements in the car directly by way of various mechanical devices. Robots are distinct from ordinary computers in their physical nature -- normal computers don't have a physical body attached to them.

In the next section, we'll look at the major elements found in most robots today.

Human Teleportation

We are years away from the development of a teleportation machine like the transporter room on Star Trek's Enterprise spaceship. The laws of physics may even make it impossible to create a transporter that enables a person to be sent instantaneously to another location, which would require travel at the speed of light.

For a person to be transported, a machine would have to be built that can pinpoint and analyze all of the 10²⁸ atoms that make up the human body. That's more than a trillion trillion atoms. This machine would then have to send this information to another location, where the person's body would be reconstructed with exact precision. Molecules couldn't be even a millimeter out of place, lest the person arrive with some severe neurological or physiological defect.

In the Star Trek episodes, and the spin-off series that followed it, teleportation was performed by a machine called a transporter. This was basically a platform that the characters stood on, while Scotty adjusted switches on the transporter room control boards. The transporter machine then locked onto each atom of each person on the platform, and used a transporter carrier wave to transmit those molecules to wherever the crew wanted to go. Viewers watching at home witnessed Captain Kirk and his crew dissolving into a shiny glitter before disappearing, rematerializing instantly on some distant planet.

If such a machine were possible, it's unlikely that the person being transported would actually be "transported." It would work more like a fax machine -- a duplicate of the person would be made at the receiving end, but with much greater precision than a fax machine. But what would happen to the original? One theory suggests that teleportation would combine genetic cloning with digitization.

In this biodigital cloning, tele-travelers would have to die, in a sense. Their original mind and body would no longer exist. Instead, their atomic structure would be recreated in another location, and digitization would recreate the travelers' memories, emotions, hopes and dreams. So the travelers would still exist, but they would do so in a new body, of the same atomic structure as the original body, programmed with the same information.

But like all technologies, scientists are sure to continue to improve upon the ideas of teleportation, to the point that we may one day be able to avoid such harsh methods. One day, one of your descendents could finish up a work day at a space office above some far away planet in a galaxy many light years from Earth, tell his or her wristwatch that it's time to beam home for dinner on planet X below and sit down at the dinner table as soon as the words leave his mouth.

Teleportation: Recent Experiments

In 1998, physicists at the California Institute of Technology (Caltech), along with two European groups, turned the IBM ideas into reality by successfully teleporting a photon, a particle of energy that carries light. The Caltech group was able to read the atomic structure of a photon, send this information across 3.28 feet (about 1 meter) of coaxial cable and create a replica of the photon. As predicted, the original photon no longer existed once the replica was made.

In performing the experiment, the Caltech group was able to get around the Heisenberg Uncertainty Principle, the main barrier for teleportation of objects larger than a photon. This principle states that you cannot simultaneously know the location and the speed of a particle. But if you can't know the position of a particle, then how can you teleport it? In order to teleport a photon without violating the Heisenberg Principle, the Caltech physicists used a phenomenon known as entanglement. In entanglement, at least three photons are needed to achieve quantum teleportation:

• Photon A: The photon to be teleported
• Photon B: The transporting photon
• Photon C: The photon that is entangled with photon B
  

If researchers tried to look too closely at photon A without entanglement, they would bump it, and thereby change it. By entangling photons B and C, researchers can extract some information about photon A, and the remaining information would be passed on to B by way of entanglement, and then on to photon C. When researchers apply the information from photon A to photon C, they can create an exact replica of photon A. However, photon A no longer exists as it did before the information was sent to photon C.

In other words, when Captain Kirk beams down to an alien planet, an analysis of his atomic structure is passed through the transporter room to his desired location, where a replica of Kirk is created and the original is destroyed.

In 2002, researchers at the Australian National University successfully teleported a laser beam.

The most recent successful teleportation experiment took place on October 4, 2006 at the Niels Bohr Institute in Copenhagen, Denmark. Dr. Eugene Polzik and his team teleported information stored in a laser beam into a cloud of atoms. According to Polzik, "It is one step further because for the first time it involves teleportation between light and matter, two different objects. One is the carrier of information and the other one is the storage medium" [CBC]. The information was teleported about 1.6 feet (half a meter).

Quantum teleportation holds promise for quantum computing. These experiments are important in developing networks that can distribute quantum information. Professor Samuel Braunstein, of the University of Wales, Bangor, called such a network a "quantum Internet." This technology may be used one day to build a quantum computer that has data transmission rates many times faster than today's most powerful computers.

Teleportation

Ever since the wheel was invented more than 5,000 years ago, people have been inventing new ways to travel faster from one point to another. The chariot, bicycle, automobile, airplane and rocket have all been invented to decrease the amount of time we spend getting to our desired destinations. Yet each of these forms of transportation share the same flaw: They require us to cross a physical distance, which can take anywhere from minutes to many hours depending on the starting and ending points.

But what if there were a way to get you from your home to the supermarket without having to use your car, or from your backyard to the International Space Station without having to board a spacecraft? There are scientists working right now on such a method of travel, combining properties of telecommunications and transportation to achieve a system called teleportation. In this article, you will learn about experiments that have actually achieved teleportation with photons, and how we might be able to use teleportation to travel anywhere, at anytime.

Teleportation involves dematerializing an object at one point, and sending the details of that object's precise atomic configuration to another location, where it will be reconstructed. What this means is that time and space could be eliminated from travel -- we could be transported to any location instantly, without actually crossing a physical distance.

Many of us were introduced to the idea of teleportation, and other futuristic technologies, by the short-lived Star Trek television series (1966-69) based on tales written by Gene Roddenberry. Viewers watched in amazement as Captain Kirk, Spock, Dr. McCoy and others beamed down to the planets they encountered on their journeys through the universe.

In 1993, the idea of teleportation moved out of the realm of science fiction and into the world of theoretical possibility. It was then that physicist Charles Bennett and a team of researchers at IBM confirmed that quantum teleportation was possible, but only if the original object being teleported was destroyed. This revelation, first announced by Bennett at an annual meeting of the American Physical Society in March 1993, was followed by a report on his findings in the March 29, 1993 issue of Physical Review Letters. Since that time, experiments using photons have proven that quantum teleportation is in fact possible.

How Radar Works

Radar is something that is in use all around us, although it is normally invisible. Air traffic control uses radar to track planes both on the ground and in the air, and also to guide planes in for smooth landings. Police use radar to detect the speed of passing motorists. NASA uses radar to map the Earth and other planets, to track satellites and space debris and to help with things like docking and maneuvering. The military uses it to detect the enemy and to guide weapons.

Meteorologists use radar to track storms, hurricanes and tornadoes. You even see a form of radar at many grocery stores when the doors open automatically! Obviously, radar is an extremely useful technology.

When people use radar, they are usually trying to accomplish one of three things:
Detect the presence of an object at a distance - Usually the "something" is moving, like an airplane, but radar can also be used to detect stationary objects buried underground. In some cases, radar can identify an object as well; for example, it can identify the type of aircraft it has detected.

Detect the speed of an object - This is the reason why police use radar.

Map something - The space shuttle and orbiting satellites use something called Synthetic Aperture Radar to create detailed topographic maps of the surface of planets and moons.
All three of these activities can be accomplished using two things you may be familiar with from everyday life: echo and Doppler shift. These two concepts are easy to understand in the realm of sound because your ears hear echo and Doppler shift every day. Radar makes use of the same techniques using radio waves.

Echo and Doppler Shift

Echo is something you experience all the time. If you shout into a well or a canyon, the echo comes back a moment later. The echo occurs because some of the sound waves in your shout reflect off of a surface (either the water at the bottom of the well or the canyon wall on the far side) and travel back to your ears. The length of time between the moment you shout and the moment that you hear the echo is determined by the distance between you and the surface that creates the echo.

Doppler shift is also common. You probably experience it daily (often without realizing it). Doppler shift occurs when sound is generated by, or reflected off of, a moving object. Doppler shift in the extreme creates sonic booms (see below). Here's how to understand Doppler shift (you may also want to try this experiment in an empty parking lot). Let's say there is a car coming toward you at 60 miles per hour (mph) and its horn is blaring. You will hear the horn playing one "note" as the car approaches, but when the car passes you the sound of the horn will suddenly shift to a lower note. It's the same horn making the same sound the whole time. The change you hear is caused by Doppler shift.

Here's what happens. The speed of sound through the air in the parking lot is fixed. For simplicity of calculation, let's say it's 600 mph (the exact speed is determined by the air's pressure, temperature and humidity). Imagine that the car is standing still, it is exactly 1 mile away from you and it toots its horn for exactly one minute. The sound waves from the horn will propagate from the car toward you at a rate of 600 mph. What you will hear is a six-second delay (while the sound travels 1 mile at 600 mph) followed by exactly one minute's worth of sound.



Now let's say that the car is moving toward you at 60 mph. It starts from a mile away and toots it's horn for exactly one minute. You will still hear the six-second delay. However, the sound will only play for 54 seconds. That's because the car will be right next to you after one minute, and the sound at the end of the minute gets to you instantaneously. The car (from the driver's perspective) is still blaring its horn for one minute. Because the car is moving, however, the minute's worth of sound gets packed into 54 seconds from your perspective. The same number of sound waves are packed into a smaller amount of time. Therefore, their frequency is increased, and the horn's tone sounds higher to you. As the car passes you and moves away, the process is reversed and the sound expands to fill more time. Therefore, the tone is lower.

You can combine echo and doppler shift in the following way. Say you send out a loud sound toward a car moving toward you. Some of the sound waves will bounce off the car (an echo). Because the car is moving toward you, however, the sound waves will be compressed. Therefore, the sound of the echo will have a higher pitch than the original sound you sent. If you measure the pitch of the echo, you can determine how fast the car is going.

Understanding Radar

We have seen that the echo of a sound can be used to determine how far away something is, and we have also seen that we can use the Doppler shift of the echo to determine how fast something is going. It is therefore possible to create a "sound radar," and that is exactly what sonar is. Submarines and boats use sonar all the time. You could use the same principles with sound in the air, but sound in the air has a couple of problems:
Sound doesn't travel very far -- maybe a mile at the most.
Almost everyone can hear sounds, so a "sound radar" would definitely disturb the neighbors (you can eliminate most of this problem by using ultrasound instead of audible sound).
Because the echo of the sound would be very faint, it is likely that it would be hard to detect.
Radar therefore uses radio waves instead of sound. Radio waves travel far, are invisible to humans and are easy to detect even when they are faint.


Let's take a typical radar set designed to detect airplanes in flight. The radar set turns on its transmitter and shoots out a short, high-intensity burst of high-frequency radio waves. The burst might last a microsecond. The radar set then turns off its transmitter, turns on its receiver and listens for an echo. The radar set measures the time it takes for the echo to arrive, as well as the Doppler shift of the echo. Radio waves travel at the speed of light, roughly 1,000 feet per microsecond; so if the radar set has a good high-speed clock, it can measure the distance of the airplane very accurately. Using special signal processing equipment, the radar set can also measure the Doppler shift very accurately and determine the speed of the airplane.


In ground-based radar, there's a lot more potential interference than in air-based radar. When a police radar shoots out a pulse, it echoes off of all sorts of objects -- fences, bridges, mountains, buildings. The easiest way to remove all of this sort of clutter is to filter it out by recognizing that it is not Doppler-shifted. A police radar looks only for Doppler-shifted signals, and because the radar beam is tightly focused it hits only one car.

Police are now using a laser technique to measure the speed of cars. This technique is called lidar, and it uses light instead of radio waves. See How Radar Detectors Work for information on lidar technology.

How Gas Turbine Engines Work

When you go to an airport and see the commercial jets there, you can't help but notice the huge engines that power them. Most commercial jets are powered by turbofan engines, and turbofans are one example of a general class of engines called gas turbine engines.

You may have never heard of gas turbine engines, but they are used in all kinds of unexpected places. For example, many of the helicopters you see, a lot of smaller power plants and even the M-1 Tank use gas turbines. In this article, we will look at gas turbine engines to see what makes them tick!

A Little Background
There are many different kinds of turbines:
You have probably heard of a steam turbine. Most power plants use coal, natural gas, oil or a nuclear reactor to create steam. The steam runs through a huge and very carefully designed multi-stage turbine to spin an output shaft that drives the plant's generator.
Hydroelectric dams use water turbines in the same way to generate power. The turbines used in a hydroelectric plant look completely different from a steam turbine because water is so much denser (and slower moving) than steam, but it is the same principle.
Wind turbines, also known as wind mills, use the wind as their motive force. A wind turbine looks nothing like a steam turbine or a water turbine because wind is slow moving and very light, but again, the principle is the same.

A gas turbine is an extension of the same concept. In a gas turbine, a pressurized gas spins the turbine. In all modern gas turbine engines, the engine produces its own pressurized gas, and it does this by burning something like propane, natural gas, kerosene or jet fuel. The heat that comes from burning the fuel expands air, and the high-speed rush of this hot air spins the turbine.


Advantages and Disadvantages
So why does the M-1 tank use a 1,500 horsepower gas turbine engine instead of a diesel engine? It turns out that there are two big advantages of the turbine over the diesel:
Gas turbine engines have a great power-to-weight ratio compared to reciprocating engines. That is, the amount of power you get out of the engine compared to the weight of the engine itself is very good.
Gas turbine engines are smaller than their reciprocating counterparts of the same power.
The main disadvantage of gas turbines is that, compared to a reciprocating engine of the same size, they are expensive. Because they spin at such high speeds and because of the high operating temperatures, designing and manufacturing gas turbines is a tough problem from both the engineering and materials standpoint. Gas turbines also tend to use more fuel when they are idling, and they prefer a constant rather than a fluctuating load. That makes gas turbines great for things like transcontinental jet aircraft and power plants, but explains why you don't have one under the hood of your car.

Heat Treatment of Steel

We can alter the characteristics of steel in various ways. In the first place, steel which contains very little carbon, will be milder than steel which contains a higher percentage of carbon, up to the limit of about 0.5%. Secondly, we can heat the steel above a certain critical temperature, and then allowit to cool at different rates. At this critical temperature, changes to begin to take place in the molecular atructure of the metal. In the process known as annealing, we heat the steel above the critical temperature and permit it to coolvery slowly. This causes the metal to become softer than before, and much easier to machine. Annealing has a second advantage. It helps to relieve any internal stresses which exist in the metal. These stresses are liadle to accur through hammering or worting the metal, or through rapid cooling. Metal which we cause to cool rapidly contracts more rapidly on the outside than on the inside. This produces unequal contractions, which may give rise to distortion or cracking. Metal which cools slowly is less liable to have these internal stresses than metal which cools quickly.
On the other hand, we can make steel harder by rapid coolin. We heat it up beyond the critical temperature, and then quench it in wateror some ather liquid. The rapid temperature drop fixes the structural change in the steel which occured at the critical temperature, and makes it very hard. But a bar of this hardened steel is more liable to fracturethan normal steel. We therefore heat it again to temperature below the critical temperature , and cool it slowly. This treatment is called tempering. It helps to relieve the internal stresses, and makes the steel less brittle than before. The properties of tempered steel enable us to use it in the manufacture of tools which need a fairly hard steel. High carbon steel is harder than tempered steel, but it is much more difficult to work.
These heat treatments take places during the various shaping operations. We can obtain bars and sheets of by rolling the metal through huge rolls in a rolling-mill. The roll pressures must be much greater for cold rolling than for hot rolling, but cold rolling enables the operators to produce rolls of great accuracy and uniformity, and with a better surface finish. Other shaping operations include drawing into wire, casting in moulds, and forging.

Modes of deformation

If, by convention, we assign the major principal direction 1 to the direction of the greatest (most positive) principal stress and consequently greatest principal strain, then all points will be to the left of the right-hand diagonal in Figure (a), i.e. left of the strain path in

Figure:(a) The strain diagram showing the different deformation modes corresponding to different strain ratios. (b) Equibiaxial stretching at the pole of a stretched dome. (c) Deformation in plane strain in the side-wall of a long part. (d) Uniaxial extension of the edge of an extruded hole. (e) Drawing or pure shear in the flange of a deep-drawn cup, showing a grid circle expanding in one direction and contracting in the other. (f) Uniaxial compression at the edge of a deep-drawn cup. (g) The different proportional strain paths shown in Figure plotted in an engineering strain diagram.

which β = 1. As stated above, the principal tension and principal stress in the direction, 1, will always be tensile or positive, i.e. σ1 ≥ 0. For the extreme case in which σ1 = 0 we find from Equations 2.6 and 2.14, that α = −∞ and β = −2. Therefore all possible straining paths in sheet forming processes will lie between 0A and 0E in Figure(a) and the strain ratio will be in the range −2 ≤ β ≤ 1.

Equal biaxial stretching, β = 1
The path 0A indicates equal biaxial stretching. Sheet stretched over a hemispherical punch will deform in this way at the centre of the process shown in Figure (b). The membrane strains are equal in all directions and a grid circle expands, but remains circular. As β = 1, the thickness strain is ε3 = −2ε1, so that the thickness decreases more rapidly with respect to ε1 than in any other process. Also from Equation 2.19(c), the effective strain is ε = 2ε1 and the sheet work-hardens rapidly with respect to ε1.

Plane strain, β = 0
In this process illustrated by path, 0B, in Figure (a), the sheet extends only in one direction and a circle becomes an ellipse in which the minor axis is unchanged. In long, trough-like parts, plane strain is observed in the sides as shown in Figure (c). It will be shown later that in plane strain, sheet is particularly liable to failure by splitting.

Uniaxial tension, β = −1/2
The point C in Figure (a) is the process in a tensile test and occurs in sheet when the minor stress is zero, i.e. when σ2 = 0. The sheet stretches in one direction and contracts in the other. This process will occur whenever a free edge is stretched as in the case of hole extrusion in Figure (d).

Constant thickness or drawing, β = −1
In this process, point D, membrane stresses and strains are equal and opposite and the sheet deforms without change in thickness. It is called drawing as it is observed when sheet is drawn into a converging region. The process is also called pure shear and occurs in the flange of a deep-drawn cup as shown in Figure (e). From Equation 3.1(b), the thickness strain is zero and from Equation 2.19(c) the effective strain is ε = 2/√3ε1 = 1.155ε1 and work-hardening is gradual. Splitting is unlikely and in practical forming operations, large strains are often encountered in this mode.

Uniaxial compression, β = −2
This process, indicated by the point E, is an extreme case and occurs when the major stress σ1 is zero, as in the edge of a deep-drawn cup, Figure (f). The minor stress is compressive, i.e. σ2 = −σf and the effective strain and stress are ε = −ε2 and σ = −σ2 respectively. In this process, the sheet thickens and wrinkling is likely.

Thinning and thickening
Plotting strains in this kind of diagram, Figure (a), is very useful in assessing sheet forming processes. Failure limits can be drawn also in such a space and this is described in a subsequent chapter. The position of a point in this diagram will also indicate how thickness is changing; if the point is to the right of the drawing line, i.e. if β > −1, the sheet will thin. For a point below the drawing line, i.e. β < −1, the sheet becomes thicker. The engineering strain diagram
In the sheet metal industry, the information in Figure (a) is often plotted in terms of the engineering strain. In Figure (g), the strain paths for constant true strain ratio paths have been plotted in terms of engineering strain. It is seen that many of these proportional processes do not plot as straight lines. This is a consequence of the unsuitable nature of engineering strain as a measure of deformation and in this work, true strains will be used in most instances. Engineering strain diagrams are still widely used and it is advisable to be familiar with both forms. In this work, true strain diagrams will be used unless specifically stated.

Uniform sheet deformation processes

An instant in the plane stress deformation of a work-hardening material was considered. We now apply the theory to some region of a sheet undergoing uniform, proportional deformation as shown in Figure . If the undeformed sheet, of initial thickness t0 is marked with a grid of circles of diameter d0 or a square mesh of pitch d0 as shown in Figure (a), then during uniform deformation, the circles will deform to ellipses of major and minor axes d1 and d2 respectively. If the square grid is aligned with the principal directions, it will become rectangular as shown in Figure (b). The thickness is denoted by t . At the instant shown in Figure (b), the deformation stresses are σ1 and σ2.

Tension as a measure of force in sheet forming
In sheet processes, deformation occurs as the result of forces transmitted through the sheet. The force per unit width of sheet is the product of stress and thickness and in Figure (c) is represented by, T = σt where, T , is known as the tension, traction or stress resultant. Because this is the product of the current thickness t as well as the current stress σ, it is the appropriate measure of force and will be used throughout this work in modelling processes. The term, tension, will be used even though this suffers from the disadvantage that the force is not always
a tensile force. If the tension is negative, it indicates a compressive force. This is not a serious problem as in plane stress sheet forming, almost without exception, one tension will be positive, i.e. the sheet is always pulled in one direction. It is impractical to forms sheet by pushing on the edge; the expression used by practical sheet formers is that ‘you cannot push on the end of a rope’. In the convention used here, the principal direction 1 is that in which the principal stress has the greatest (most positive) value, and the major tension T1 = σ1t will always be positive. In stretching processes, the minor tension T2 = σ2t is tensile or positive. In other processes, the minor tension could be compressive and in some cases the thickness will increase. If T2 is compressive and large in magnitude, wrinkling may be a problem. In discussing true stress in Section , it was shown that for most real materials, strain-hardening continues, although at a diminishing rate, and true stress does not reach a maximum. As tension includes thickness, which in many processes will diminish, T may reach a maximum; this limits the sheet’s ability to transmit load and is one of the reasons for considering tension in any analysis.

Strain distributions
In the study of any process, we usually determine first the strain over the part. This can be done by measuring a grid as in Figure 3.1, or by analysis of the geometric constraint exerted on the part. An example is the deep drawing process in Figure (a) and in the Introduction, Figure I.9. As the process is symmetric about the axis, we need only consider the strain at points on a line as shown in Figure (b). Plotting these strains in the principal strain space, Figure (c), gives the locus of strains for a particular stage in the process. As the process continues, this locus will expand, but not necessarily uniformly; some points may stop straining, while others go on to reach a process limit. For any process, there will be a characteristic strain pattern, as shown in Figure (c). This is sometimes known as the ‘strain signature’. Considerable information can be
obtained from such a diagram and the way it is analysed is outlined in the following section.

Strain diagram
The individual points on the strain locus in second Figure (c) can be obtained from measurements of a grid circles as shown in first Figure . (If a square grid is used, the analysis method is outlined in Appendix A.2.) If the major and minor axes are measured and the current thickness determined, the analysis is as follows.

Maximum shear stress

On the faces of the principal element on the left-hand side of Figure 2.4, there are no shear stresses. On a face inclined at any other angle, both normal and shear stresses will act. On faces of different orientation it is found that the shear stresses will locally reach a maximum for three particular directions; these are the maximum shear stress planes and are illustrated in Figure 2.4. They are inclined at 45◦ to the principal directions and the maximum shear stresses can be found from the Mohr circle of stress, Figure . Normal stresses also act on these maximum shear stress planes, but these have not been shown in the diagram.

The three maximum shear stresses for the element are
τ1 =(σ1 − σ2)/2 ; τ2 =(σ2 − σ3)/2 ; τ3 =(σ3 − σ1)/2
From the discussion above, it might be anticipated that yielding would be dependent on the shear stresses in an element and the current value of the flow stress; i.e. that a yielding condition might be expressed as
f (τ1, τ2, τ3) = σf

Yielding in plane stress

The stresses required to yield a material element under plane stress will depend on the current hardness or strength of the sheet and the stress ratio α. The usual way to define the strength of the sheet is in terms of the current flow stress σf. The flow stress is the stress at which the material would yield in simple tension, i.e. if α = 0. This is illustrated in the true stress–strain curve in Figure. Clearly σf depends on the amount of deformation to which the element has been subjected and will change during the process. For the moment, we shall consider only one instant during deformation and, knowing the current value of σf the objective is to determine, for a given value of α, the values of σ1 and σ2 at which the element will yield, or at which plastic flow will continue for a small increment. We consider here only the instantaneous conditions in which the strain increment is so small that the flow stress can be considered constant. In Chapter 3 we extend this theory for continuous deformation.
There are a number of theories available for predicting the stresses under which a material element will deform plastically. Each theory is based on a different hypothesis about material behaviour, but in this work we shall only consider two common models and apply them to the plane stress process described by Equations ε1; ε2 = βε1; ε3 = −(1 + β)ε1. Over the years, many researchers have conducted experiments to determine how materials yield. While no single theory agrees exactly with experiment, for isotropic materials either of the models presented here are sufficiently accurate for approximate models.

With hindsight, common yielding theories can be anticipated from knowledge of the nature of plastic deformations in metals. These materials are polycrystalline and plastic flow occurs by slip on crystal lattice planes when the shear stress reaches a critical level. To a first approximation, this slip which is associated with dislocations in the lattice is insensitive to the normal stress on the slip planes. It may be anticipated then that yielding will be associated with the shear stresses on the element and is not likely to be influenced by the average stress or pressure. It is appropriate to define these terms more precisely.

General sheet processes (plane stress)

In contrast with the tensile test in which two of the principal stresses are zero, in a typical sheet process most elements will deform under membrane stresses σ1 and σ2, which are both non-zero. The third stress, σ3, perpendicular to the surface of the sheet is usually quite small as the contact pressure between the sheet and the tooling is generally very much lower than the yield stress of the material. As indicated above, we will make the simplifying assumption that it is zero and assume plane stress deformation, unless otherwise stated. If we also assume that the same conditions of proportional, monotonic deformation apply as for the tensile test, then we can develop a simple theory of plastic deformation of sheet that is reasonably accurate. We can illustrate these processes for an element as shown in Figure (a) for the uniaxial tension and Figure (b) for a general plane stress sheet process.

Stress and strain ratios

It is convenient to describe the deformation of an element, as in Figure (b), in terms of either the strain ratio β or the stress ratio α. For a proportional process, which is the only kind we are considering, both will be constant. The usual convention is to define the principal directions so that σ1 > σ2 and the third direction is perpendicular to the surface where σ3 = 0. The deformation mode is thus:

ε1 ; ε2 = βε1 ; ε3 = −(1 + β)ε1

σ1 ; σ2 = ασ1 ; σ3 = 0

The constant volume condition is used to obtain the third principal strain. Integrating thestrain increments in dε1 + dε2 + dε3 = 0 shows that this condition can be expressed in terms of the true or natural strains:
i.e. the sum of the natural strains is zero.
For uniaxial tension, the strain and stress ratios are β = −1/2 and α = 0.

True, natural or logarithmic strains

It may be noted that in the tensile test the following conditions apply:
*the principal strain increments all increase smoothly in a constant direction, i.e. dε1 always increases positively and does not reverse; this is termed a monotonic process;
*during the uniform deformation phase of the tensile test, from the onset of yield to the maximum load and the start of diffuse necking, the ratio of the principal strains remains constant, i.e. the process is proportional; and
*the principal directions are fixed in the material, i.e. the direction 1 is always along the axis of the test-piece and a material element does not rotate with respect to the principal directions.
If, and only if, these conditions apply, we may safely use the integrated or large strains defined in Principal strain increments. For uniaxial deformation of an isotropic material, these strains are
ε1 = ln(l/l0) ; ε2 = ln(w/w0 = (−1/2)ε1 ; ε3 = ln(t/to) = (−1/2)ε1

Stress and strain ratios (isotropic material)

If we now restrict the analysis to isotropic materials, where identical properties will be measured in all directions, we may assume from symmetry that the strains in the width and thickness directions will be equal in magnitude and hence, from dε1 + dε2 + dε3 = 0,

dε2 = dε3 = −(1/2)dε1

(In the previous chapter we considered the case in which the material was anisotropic where dε2 = Rdε3 and the R-value was not unity. We can develop a general theory for anisotropic deformation, but this is not necessary at this stage.) We may summarize the tensile test process for an isotropic material in terms of the strain increments and stresses in the following manner:

dε1 =dl/l ; dε2 = −(1/2)dε1 ; dε3 = −(1/2)dε1
and

σ1 =P/A ; σ2 = 0 ; σ3 = 0

Constant volume (incompressibility) condition

It has been mentioned that plastic deformation occurs at constant volume so that these strain increments are related in the following manner. With no change in volume, the differential of the volume of the gauge region will be zero, i.e.

d(lwt) = d(lowoto) = 0

and we obtain

(dl . wt) + (dw . lt) + (dt . lw) = 0

or, dividing by lwt,

(dl/l )+(dw/w) +(dt/t) = 0

i.e.

dε1 + dε2 + dε3 = 0

Thus for constant volume deformation, the sum of the principal strain increments is zero.

Principal strain increments

During any small part of the process, the principal strain increment along the tensile axis is given by Equation 1.10 and is
dε1 =dl/l

i.e. the increase in length per unit current length. Similarly, across the strip and in the through-thickness direction the strain increments are
dε2 =dw/w and dε3 =dt/t

Uniaxial tension

We consider an element in a tensile test-piece in uniaxial deformation and follow the process from an initial small change in shape. Up to the maximum load, the deformation is uniform and the element chosen can be large and, in Figure 2.1, we consider the whole gauge section. During deformation, the faces of the element will remain perpendicular to each other as it is, by inspection, a principal element, i.e. there is no shear strain associated with the principal directions, 1, 2 and 3, along the axis, across the width and through the thickness, respectively.

Tensile test

For historical reasons and because the test is easy to perform, many familiar material properties are based on measurements made in the tensile test. Some are specific to the test and cannot be used mathematically in the study of forming processes, while others are fundamental properties of more general application. As many of the specific, or onfundamental tensile test properties are widely used, they will be described at this stage and some description given of their effect on processes, even though this can only be done in a qualitative fashion.
A tensile test-piece is shown in Figure 1.1. This is typical of a number of standard test-pieces having a parallel, reduced section for a length that is at least four times the width, w0. The initial thickness is t0 and the load on the specimen at any instant, P, is measured by a load cell in the testing machine. In the middle of the specimen, a gauge length l0 is monitored by an extensometer and at any instant the current gauge length is l and the extension is l = l − l0. In some tests, a transverse extensometer may also be used to measure the change in width, i.e. w= w − w0. During the test, load and extension will be recorded in a data acquisition system and a file created; this is then analysed and various material property diagrams can be created. Some of these are described below.

The load–extension diagram
Figure 1.2 shows a typical load–extension diagram for a test on a sample of drawing quality steel. The elastic extension is so small that it cannot be seen. The diagram does not represent basic material behaviour as it describes the response of the material to a particular process, namely the extension of a tensile strip of given width and thickness. Nevertheless it does give important information. One feature is the initial yielding load, Py, at which plastic deformation commences. Initial yielding is followed by a region in which the deformation in the strip is uniform and the load increases. The increase is due to strain-hardening, which is a phenomenon exhibited by most metals and alloys in the soft condition whereby the strength or hardness of the material increases with plastic deformation. During this part of the test, the cross-sectional area of the strip decreases while the length increases; a point is reached when the strain-hardening effect is just balanced by the rate of decrease in area and the load reaches a maximum Pmax .. Beyond this, deformation in the strip ceases to be uniform and a diffuse neck develops in the reduced section; non-uniform extension continues within the neck until the strip fails.

The extension at this instant is lmax ., and a tensile test property known as the total
elongation can be calculated; this is defined by

Etot. =[(lmax − l0)/l0].100%


The engineering stress–strain curve
Prior to the development of modern data processing systems, it was customary to scale the load–extension diagram by dividing load by the initial cross-sectional area, A0 = w0t0, and the extension by l0, to obtain the engineering stress–strain curve. This had the advantage that a curve was obtained which was independent of the initial dimensions of the test-piece, but it was still not a true material property curve. During the test, the cross-sectional area will diminish so that the true stress on the material will be greater than the engineering stress. The engineering stress–strain curve is still widely used and a number of properties are derived from it. Figure 1.3(a) shows the engineering stress strain curve calculated from the load, extension diagram in Figure 1.2.

Engineering stress is defined as ; σ = P/A0 (1.2)

and engineering strain as ; e = (l /l0). 100% (1.3)

In this diagram, the initial yield stress is; (σf)0 = Py/A0 (1.4)

The maximum engineering stress is called the ultimate tensile strength or the tensile
strength and is calculated as ; T S = Pmax/A0 (1.5)

As already indicated, this is not the true stress at maximum load as the cross-sectional area is no longer A0. The elongation at maximum load is called the maximum uniform elongation, Eu. If the strain scale near the origin is greatly increased, the elastic part of the curve would be seen, as shown in Figure 1.3(b). The strain at initial yield, ey, as mentioned, is very small, typically about 0.1%. The slope of the elastic part of the curve is the elastic modulus, also called Youngs modulus:
E = (σf)0 / ey (1.6)

If the strip is extended beyond the elastic limit, permanent plastic deformation takes place; pon unloading, the elastic strain will be recovered and the unloading line is parallel to the initial lastic loading line. There is a residual plastic strain when the load has been removed as shown in Figure 1.3(b).

In some materials, the transition from elastic to plastic deformation is not sharp and it is difficult to establish a precise yield stress. If this is the case, a proof stress may be quoted. This is the stress to produce a specified small plastic strain – often 0.2%, i.e. about twice the elastic straint yield. Proof stress is determined by drawing a line parallel to the elastic loading line which is offset by the specified amount, as shown in Figure 1.3(c). Certain teels are susceptible to strain ageing and will display the yield phenomena illustrated in Figure 1.4. This may be seen in some hot-dipped galvanized steels and in bake-hardenable steels used in autobody panels. Ageing has the effect of increasing the initial yielding stress to the upper yield stress σU; beyond this, yielding occurs in a discontinuous form. In the tensile test-piece, discrete bands of deformation called L¨uder’s lines will traverse the strip under a constant stress that is lower than the upper yield stress; this is known as the lower yield stress σL. At the end of this discontinuous flow, uniform deformation associated with strain-hardening takes place. The amount of discontinuous strain is called the yield point elongation (YPE). Steels that have significant yield point elongation, more than about 1%, are usually unsuitable for forming as they do not deform smoothly and visible markings, called stretcher strains can appear on the part.

***if some figures looking small, you can click on the pictures for magnify.

Material Properties Introduction

The most important criteria in selecting a material are related to the function of thenpart – qualities such as strength, density, stiffness and corrosion resistance. For sheet material, the ability to be shaped in a given process, often called its formability, should also be considered. To assess formability, we must be able to describe the behaviour of the sheet in a precise way and express properties in a mathematical form; we also need to know how properties can be derived from mechanical tests. As far as possible, each property should be expressed in a fundamental form that is independent of the test used to measure it. The information can then be used in a more general way in the models of various metal forming processes that are introduced in subsequent chapters. In sheet metal forming, there are two regimes of interest – elastic and plastic deformation. Forming a sheet to some shape obviously involves permanent ‘plastic’ flow and the strains in the sheet could be quite large. Whenever there is a stress on a sheet element, there will also be some elastic strain. This will be small, typically less than one part in one thousand. It is often neglected, but it can have an important effect, for example when a panel is removed from a die and the forming forces are unloaded giving rise to elastic shape changes, or ‘springback’.